This invention relates to toner dispensers for electrophotographic copiers and duplicators and, more particularly to a low toner detection and indicating system for warning a machine operator of impending exhaustion of the toner supply.
In the process of electrostatographic printing, an electrostatic charge pattern or latent image corresponding to an original document to be reproduced is recorded on an insulating medium. A viewable record is produced by developing the latent image with particles of granulated material to form a powder image thereof. Thereafter, the visible powder image is fused to the insulating medium, or transferred to a suitable support material and fused thereto. Development of the latent image is achieved by bringing a developer mix into contact therewith. Typical developer mixes generally comprise dyed or colored thermoplastic particles of granulated material known in the art as toner particles, which are mixed with carrier granules, such as ferromagnetic granules. When appropriate, toner particles are mixed with carrier granules and the toner particles are charged triboelectrically to the correct polarity. As the developer mix is brought into contact with the electrostatic latent image, the toner particles adhere thereto. However, as toner particles are depleted from the developer mix, additional toner particles (simply "toner" hereafter) must be supplied. In this way, the concentration of toner in the developer mix is maintained substantially constant.
In the operation of an electrophotographic copier or printer, it is highly desirable to have means for indicating when the supply of toner in the dispensing apparatus has become exhausted. When the supply of toner finally nears exhaustion, there may be a perceptible reduction in the density of the developed image and, therefore, a corresponding degradation in copy quality, e.g., unacceptable light copy. This latter condition is usually the first indication to an operator that the toner supply is in need of replenishment. Upon installation of a new toner supply in the form of a toner cartridge, and upon continued resumption of copying, the output copies will still be light because of the continued low toner concentration. To recover from this detoned condition, under normal dispensing, may take several days. For copiers which reproduce large documents such as the Xerox 2510, the machine, without corrective action, will never fully recover. The remedy, at this point, is to add fresh toner to the developer mix to increase the toner concentration to the required level before resuming operation. Typically, this is achieved by manually rotating an installed dispenser cartridge until the desired concentration level is reached.
It is, therefore, apparent that an early warning of impending toner exhaustion would be desirable so that toner could be replenished before being totally exhausted. Toner replacement could then be implemented at a time while the toner concentration is still at acceptable levels. There have been several approaches in the prior art to provide this low toner level sensing indication. U.S. Pat. No. 4,135,642, assigned to the same assignee as the present invention, illustrates a technique whereby an opposed lamp and photocell are located at the bottom of a toner dispensing housing. When the toner level drops to a sufficiently low point, an electrical signal is produced at the photocell which can be used to generate a visual or audible indication of low toner. A publication entitled "Toner Level Sensor", published in the Xerox Disclosure Journal, Vol. 5, No. 1, Jan./Feb. 1980, discloses a float type sensor which rides onthe toner bed within the cartridge and provides an indication, on a calibrated display, of low toner level.
Other toner art methods utilized a mechanism which is internal to the toner dispenser housing and which interacts in some manner with the toner supply. The mechanism is coupled electrically or magnetically to an electrical circuit located adjacent to, but outside of the housing. U.S. Pat. No. 3,920,155, assigned to the same assignee as the present invention discloses a sensing means 114. The housing is oscillated to dispense toner through output ports; the sensing mean extends into the toner bed and is oscillated at the same rate as the housing. When the toner level drops below a specified point, the sensing mechanism becomes free of contact with the toner and begins oscillating at a rate different than that of the housing. A detecting means exterior to the housing, is located so as to detect the different oscillation rates of the housing and the sensing mechanism and generates a signal indicating a low toner level. U.S. Pat. No. 3,979,012, also assigned to the same assignee as the present invention, discloses another low level sensing system which uses a longitudinally installed shaft member with magnetic paddles protruding through the ends. The paddles extend into the toner bed and rotate therewith until the toner drops below a certain point. At that time, the paddles come under the influence of magnets located in the interior sides of the housing and rotate to a position where a switch is closed producing a low toner signal.
The present invention is directed to a low toner level sensing and indicator system which is relatively simple to install and to operate and requires the use of only a single magnet and a single magnetically enabled switch. More particularly, the invention relates to an apparatus for dispensing toner particles including, in combination, a cylindrical rotating toner housing have a plurality of longitudinal toner dispensing ports for a toner supply contained therein, an arm assembly pivotably mounted within the housing on the central axis thereof, said arm assembly including a downwardly extending arm member which attains a plumb position when the toner level falls below a certain low level while maintaining an out of plumb position when toner is above said low level; magnetic means connected to said arm member, said magnetic means enabling a magnetic sensing circuit aligned with said magnetic means when said arm member is in the plumb position.